It is necessary in analytical systems to have fittings that create leak-tight seals. In such analytical systems, it is also desirous to have fittings which are inert relative to the sample components, which provide a flow path without inducing turbulence or mixing, and which adds minimal volume to the system.
Fitting designs which best address the aspect of added volume allow the tube ends which pass through such fittings to butt directly to each other or have bores which match the tubing bore, leaving no dead or unswept volume. Such fittings are called zero dead volume fittings. In analytical testing apparati dead volume is to be avoided as it reduces efficiencies of the test equipment, including gas columns.
Tubing used may be copper, stainless steel, or polymer based, including PEEK (Poly Ether Ether Ketone), PTFE (PolyTetraFluoroEthylene), ETFE (ethylene-tetrafluoroethylene), FEP (Flouridated Ethylene-Propylene), PFA (Perfluoroalkoxyethylene), and nylon.
One of the most common types of such zero dead volume fittings is a compression fitting. A zero dead volume compression fitting consists of a fitting body having a female fitting detail, tubing, a ferrule loosely riding on said tubing, and a male nut, also riding on said tubing. In operation the tubing is placed into the fitting body until the tubing end abutts the inner bore of the fitting body and seats correctly at the bottom of the fitting detail, the ferrule is then slid along the tubing until it engages the mating walls of the fitting body, and the nut is threadedly engaged so as to be retained in place and to fully press the ferrule against the mating wall of the fitting detail, causing the ferrule to apply pressure to the tubing and force such tubing against said bore of said fitting body.
A basic problem with such system is the number of parts, namely the fitting, the ferrule, the tubing and nut. Moreover such parts are typically small, permitting clasping by fingertips only. As can be anticipated and as is known in the prior art, as the tubing end is undeformed, the ferrule may slide off the tubing end after placement of the nut and the ferrule on the tubing but before insertion of the ferrule into the fitting body. Various manufacturers have attempted to reduce the number of parts, notably by molding a nut with a ferrule on its end. It is often desirable that equipment used for chemical analysis including the ferrule and nut be of an inert material so that the equipment does not react with the sample(s) being tested or the carrier(s) conveying the sample(s), which would contaminate the sample and produce false results. Selection of an inert material sufficient in strength to serve as the nut limits the number of materials available. Moreover as the molded ferrule contacts the fitting detail of the fitting body, tortional forces are created as friction between the molded ferrule portion and the fitting body opposes the force applied by the nut as it engages the fitting body threads. Such forces may weaken or destroy the assembly. An additional problem, distortion of the tubing, may also result from the use of such ferrule-nut assemblies. As the molded nut-ferrule assembly rotates, rotational forces are applied to the end of the tubing. Such forces may cause the tubing to twist relative to its opposition end or may result in deformation of the tubing end during rotation of the molded nut-ferrule assembly.
A further potential problem with the prior art nut and ferrule assembly may occur during removal of the prior art assembly from the fitting. During such assembly, the ferrule may resist removal from the fitting as a result of the forces applied during installation. As a result, the nut may be removed, leaving the ferrule still installed. Due to the size of the ferrule, removal from the detail in light of such installation forces can be difficult.
Therefore, it is a feature of the present invention to provide a nut and ferrule assembly which in operation reduces the number of loose parts, avoids the limitation of available materials, reduces the tortional problems associated with molded nut-ferrule assemblies, and reduces the potential for difficulties in disassembly.